Going out on my particular slant, the LRM-10 on the top of the MW:DA Bellona tank would be perfect. Just cut a notch out of the back of it, cut off the barrel, and glue.
The IWM Scorpion is very flat, so you can't use many other LRM or SRM tubes without it looking disproportional, IMHO. The Harasser turret is ... pretty big. I've got some Bulldogs modified with a larger LRM - I'll try and get a picture, as it'll give a good idea what I mean.
W.